This invention is related to techniques of optical measurement of various conditions of solid object as various thermal and physical conditions characteristic of a substrate surface.
There are many examples of industrial processes where material in various forms is necessarily heated by various techniques. One example is in heating materials for the purpose of testing them. Another is in the heat treatment of an object. A further example of is found in the semiconductor processing industry. In this latter example, silicon wafers to be processed are positioned within an enclosed chamber where they are heated by some appropriate technique, usually radio frequency or optical radiation. In one form, such a semiconductor processing chamber is made, at least partially, of an optically transparent material. Lamps outside the chamber direct a large amount of energy through its transparent walls and onto the wafer. The wafer is heated as a result of its absorption of the thermal radiation. Generally, the chamber is formed of a quartz envelope, or of stainless steel with an optical window. The heated wafer is treated by introducing appropriate gases into the chamber which react with the heated surface of the wafer.
These processes require that the temperature of the wafer be maintained within narrow limits in order to obtain good processing results. Therefore, some technique of monitoring the temperature of the wafer is required. One possibility is to contact the wafer with a conventional thermocouple, but this is precluded by poor measurement and contamination considerations when semiconductor wafers are the objects being heated. For other types of objects, such contact measurement techniques most often are precluded because of a number of practical considerations. Use of a thermocouple also often results in substantial errors because of a differing thermal mass, poor thermal contact and a difference in emittance between the thermocouple and the object being heated.
As a result, most optical heating applications use some form of a long wavelength pyrometer. This technique measures the intensity of the radiation of the semiconductor wafer or other optically heated object within a narrow wavelength band. That radiation intensity is then correlated with temperature of the object. In order to avoid error by the pyrometer receiving heating optical radiation reflected from the object being heated, the wavelength chosen for monitoring by the pyrometer is outside of the emission spectrum of heating lamps. This detected wavelength range is generally made to be significantly longer than the spectrum of the lamps.
There are several disadvantages to such existing pyrometric systems. First, a measurement made at a longer wavelength will have only a portion of the sensitivity of one made at a shorter wavelength. Second, the emissivity of silicon and other materials that are optically heated is dependent upon the temperature and wavelength at which it is measured. Third, economical photodetectors with the highest signal-to-noise ratio are those which respond to the shorter wavelength emissions. Fourth, existing optical pyrometers have a small numerical aperture and thus provide temperature measurements which are also dependent upon the degree of roughness of the object and film growth being measured. Fifth, existing economical pyrometric techniques are slow, a significant disadvantage in a rapid heating system. Sixth, measurements made through a quartz window by a typical pyrometer are subject to error as a result of a significant amount of energy that is emitted by quartz in longer wavelengths to which pyrometers are sensitive.
Therefore, it is a principal object of the present invention to provide an improved pyrometric technique of temperature and/or emissivity measurements that overcomes these shortcomings.
It is also an object of the present invention to provide non-contact techniques for monitoring and/or measuring various optical conditions of surfaces as well as thermal conditions.
There are also numerous processes involving layers of material, typically thin films, where a physical parameter, such as thickness, of the film must be measured. Usually, it is desired that a new film formed on a substrate have a desired thickness within close tolerances. Other applications involve removal of material from a layer in order to form a thin film having a precise thickness. A major application of thin film technology is also found in the manufacturing of integrated circuits, both in silicon semiconductor and gallium arsenide based technology. A typical process of forming integrated circuits in the type of heated chamber discussed above involves the formation of many films and the removal of films. It is necessary in such thin film processes to know at least when the endpoint of the film formation or removal step has been completed. It is also desirable to be able to monitor and control the process in real time by a technique that does not itself interfere with the process.
Therefore, it is also a principal object of the present invention to provide surface monitoring process having a general utility in numerous processes where thin films are utilized, such as in the manufacture of integrated circuits and in the processing of metals.
It is another object of the present invention to provide a technique for measuring the thickness of very thin films with a high degree of accuracy and high resolution.
There are also many occasions where the structure or chemical composition of a surface changes, either by accident or design, such as by its corrosion, oxidation, surface passivation, formation of rust, and the like. Generally, a layer is formed on a surface that is of a different material than that of the original surface but changes can also occur by molecules of the different material diffusing into the surface. An example of an industrial processes where this occurs is in aluminum processing where slag forms on the molten aluminum surface, or where oil is sprayed onto a surface of the aluminum being rolled. In both of these examples, the surface emissivity is unknown and changing.
Accordingly, it is a further object of the present invention to provide a technique for monitoring and measuring surface characteristics under conditions of such changes occurring.
More specifically, it is an object of the present invention to provide a method of correctly measuring a property, such as temperature, by pyrometric means despite changes in surface emissivity which unavoidably results from such surface changes.